[R] Adding a new conditional column to a list of dataframes

[Jeff Newmiller]

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I think you need to pay attention to what your function is returning... whatever is on the last line of the function is key. Try testing your function separately from the lapply to insure you get what you want. Also be sure to use braces when they are needed to define your function. 

On April 15, 2018 4:08:35 AM PDT, Allaisone 1 <Allaisone1 using hotmail.com> wrote:
>
>Hi all ..,
>
>
>I have a list of 7000 dataframes with similar column headers and I
>wanted to add a new column to each dataframe based on a certain
>condition which is the same for all dataframes.
>
>
>When I extract one dataframe and apply my code it works very well as
>follows :-
>
>
>First suppose this is my first dataframe in the list
>
>> OneDF <- Mylist[[1]]
>
>> OneDF
>
>
>ID       Pdate                  Tdate
>
>1         2010-09-30       2011-05-10
>
>2         2011-11-07       2009-09-31
>
>3         2012-01-05        2008-06-23
>
>
>To add a new column where "C" has to be written in that column only if
>the date in
>
>"Tdate" column is less than the first date(row) in "Pdate"
>column.Otherwise "NA" is written.
>
>I have written this code to do so :-
>
>
>OneDF$NewCol [ OneDF[ ,3] <  OneDF[ 1,2] ] <- "C"
>
>
>This gave me what I want as follows :-
>
>
>ID       Pdate                  Tdate                      NewCol
>
>1         2010-09-30       2011-05-10                NA
>
>2         2011-11-07       2009-09-31                  C
>
>3         2012-01-05        2008-06-23                 C
>
>
>However, when I tried to apply this code in a function and then apply
>this function
>
>to all dataframes using lapply() function , I do not get what I want.
>
>
>I wrote this function first :-
>
>
>MyFunction <- function(x) x$NewCol [ x[ ,3] <  x[ 1,2] ] <- "C"
>
>
>Then I wrote this code to apply my function to all dataframes in
>"Mylist" :
>
>
>NewList <- lapply(names(Mylist), function(x) MyFunction(Mylist[[x]]))
>
>
>This returned a list of 7000 elements and each of which contain "C''
>letter. Each
>
>dataframe has become a vector of "C'' letter which is totally away from
>what I need.
>
>I expected to see a list of my 7000 dataframes and each of which looks
>like the output
>
>I have shown above with the new column.
>
>
>I spent a lot of time trying to know what  is the mistake I have made
>in these last two codes
>
>but was not able to know the issue.
>
>
>Could you please let me know my mistake and how to correct my syntax ?
>
>
>Kind Regards
>
>Allaisone
>
>
>
>
>
>
>
>
>	[[alternative HTML version deleted]]
>
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-- 
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